[R] Repeating the same calculation across multiple pairs of variables

[Joshua Wiley]

Hi Al,

Assuming that the order of the matrices resulting from selecting
"avars" and "bvars" is identical (it is at least in the example you
gave), then you can do:

dat <- data.frame(a1=1:10, a2=11:20, a3=21:30, b1=101:110, b2=111:120,
b3=121:130)

avars <- paste("a", 1:3, sep = '')
bvars <- paste("b", 1:3, sep = '')
cvars <- paste("c", 1:3, sep = '')

dat[, cvars] <- dat[, avars] / dat[, bvars]

If you are using character strings for the names, you need to use [
rather than $.  For documentation, see ?"["

Hope this helps,

Josh

On Fri, Mar 4, 2011 at 4:37 PM, Al Roark <hrbuilder at hotmail.com> wrote:
>
> Hi all,
>
> I frequently encounter datasets that require me to repeat the same calculation across many variables. For example, given a dataset with total employment variables and manufacturing employment variables for the years 1990-2010, I might have to calculate manufacturing's share of total employment in each year. I find it cumbersome to have to manually define a share for each year and would like to know how others might handle this kind of task.
>
> For example, given the data frame:
>
> df<-data.frame(a1=1:10, a2=11:20, a3=21:30, b1=101:110, b2=111:120, b3=121:130)
>
> I'd like to append new variables--c1, c2, and c3--to the data frame that are the result of a1/b1, a2/b2, and a3/b3, respectively.
>
> When there are only a few of these variables, I don't really have a problem, but it becomes a chore when the number of variables increases. Is there a way I can do this kind of processing using a loop? I tried defining a vector to hold the names for the "c variables" (e.g. c1,c2, ... cn) and creating new variables in a loop using code like:
>
> avars<-c("a1","a2","a3")
> bvars<-c("b1","b2","b3")
> cvars<-c("c1","c2","c3")
> for(i in 1:3){
>  df$cvars[i]<-df$avars[i]/df$bvars[i]
> }
>
> But the variable references don't resolve properly with this particular syntax.
>
> Any help would be much appreciated. Cheers.
>        [[alternative HTML version deleted]]
>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/